This invention relates to the art including injection molding machines and apparatus for thermoplastic and thermosetting materials.
Such machines are well known and have heretofore, in the case of machines for thermosetting materials, comprised (1) a feed chute for feeding particulate plastic particles, pellets, or the like from a supply hopper therefor, to (2) a heating cylinder in which the plastic particles are forced through a high temperature zone in the cylinder by means of (3) a plunger, sufficiently high pressure being exerted therein to force the plastic particles through the heat zone to become molten without being appreciably polymerized so that the molten or viscous mass can pass through the orifice of (4) a nozzle and thence into a mold which can be provided with clamps so that the viscous plastic injected thereinto can be contained.
Machines for injection molding thermoplastics are generally similar to those briefly described above for thermosetting materials especially as regards the (1) feed hopper and chute, (2) the heating cylinder, (3) the ram or plunger, and (4) the nozzle. Additionally, thermoplastics are injection molded by machines in which the cylinder is heated by heat bands rather than by heating the nozzle by energization of a low-voltage transformer in order that instant heating of the viscous plastic rather than gradual heating thereof can be accomplished. Also, one often employs a "torpedo", so-called because of its shape, in the cylinder of the machine for molding thermoplastics, the torpedo acting as a spreader to divert the particulate molding powder into narrow, constricted channels where the particles of powder can be more uniformly heated and softened.
As noted above, machines for injection molding of both types of plastic materials employ plungers or rams. Some are hydraulically activated others are mechanically driven. Some machines are known which use reciprocating screws to force under pressure the powder form of plastic through the heated cylinder or nozzle, all of which prior known mechanisms of propulsion being subject to a common disadvantage in that extremely small tolerances of machining are required to produce an efficiently operating machine. All such devices are subject to wear in use whereby the close tolerances are destroyed rendering such machines relatively inefficient or inoperative.
Additionally, the initial cost of manufacture of such plungers, rams, screws and the like is very high as is the replacement cost when the part is worn and can no longer function effectively.
It is therefore a principal object of this invention to provide a machine for injection molding of thermoplastic or thermosetting materials that operates effectively and efficiently without employing a plunger, ram, screw or similar solid means of propelling particulate plastic materials through the heating cylinder of the machine.
Another object of this invention is to provide such an injection molding machine wherein air under pressure constitutes the only means other than gravity for moving particulate plastic material through the heating cylinder of the machine thereby obviating the need for solid means such as rams, plungers or reciprocating screws, for performing this function, thus to eliminate the disadvantages inherent in such solid moving means as plungers, rams and screws.
It is known to use air pressure in the molding of plastics. Such use of air pressure is common in blow molding technology. However, it will be noted that in blow molding, whether it be (1) extrusion or direct blow molding, (2) indirect blow molding either by (a) the so-called tubing method or (b) the two sheet method, or (3) injection blow molding, the employment of air pressure occurs in connection with the parison or hollow plastic member that has preliminarily been formed primarily by an extrusion process in all of which known machines or apparatus plastic powder or particulate material is moved from the feed end adjacent a hopper to and through the heating and melting region by rams, plungers or screws. No reference or suggestion is made in such standard texts as "Exploring the World of Plastics", by Gerald L. Steele, McKnight Publishing Company, Bloomington, Illinois, 1st Edition, 1977, pp. 108-131 or "Modern Plastic Encyclopedia", edited by Joan Agranoff, McGraw Hill Publications Company, New York, N.Y., October 1977, No. 10A, pp. 230-234 to or for the use of air pressure as the motive force to replace rams, plungers or screws in injection molding machines or, indeed, in extrusion apparatus.